Figure 4 | Scientific Reports

Figure 4

From: Self-propelled droplet transport on shaped-liquid surfaces

Figure 4

Strength of gradient induced self-propulsion. (a) \({5}\, \upmu {\hbox {L}}\) water droplet propelled uphill on a textured liquid surface tilted at an angle \(\beta = 2.4^{\circ }\). The scale bar is 1 mm. (b) Critical sliding angles \(\beta\) as a function of rail fraction gradient \(\alpha\), for \({5}\, \upmu {\hbox {L}}\) water droplets. (blue) circles stand for the critical tilting angles \(\beta _1\) at which droplets stop moving uphill and (brown) squares stand for the critical tilting angles \(\beta _2\) at which droplets stop moving downhill. Dim areas represent the standard deviation calculated on 12 measurements. Between those two critical angles, the driving force is too small to overcome the pinning force and the droplet remains motionless. The (blue) triangle highlights the configuration depicted in (a). (c) Driving force \(F_d\) and pinning force \(F_p\) calculated from the critical angle measurements presented in (b). (orange) circles are the driving force for liquid (filled symbols) and super-hydrophobic (open symbol) surfaces. (green) squares are the pinning force for liquid (filled symbols) and super-hydrophobic (open symbol) surfaces. Dim areas represent the standard deviation propagated from the measurement of the critical angles. Dashed (orange) and dotted (green) lines are the theoretical predictions of respectively the driving force and the pinning force (see text).

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